Ultra-violet light curable (UV) and electron beam energy curable (EB) lithographic inks are well established market products in the printing ink or graphic arts market. Examples of energy curable lithographic inks are disclosed in U.S. Pat. No. 5,985,984 and U.S. Pat. No. 6,316,517, which is incorporated herein by reference. Their major benefits are good physical and, chemical resistance properties achieved immediately after curing. Instantaneous cure of UV and EB inks allows for such uses as converting of folding cartons in-line without damaging a printed image (scratches, flaking off, etc.). However, conventional UV/EB lithographic inks suffer from a number of lithographic printing deficiencies such as narrow water window, poor emulsion quality which results in toning and scumming, and a tendency to pile on the offset printing blankets due to poor ink transfer and release which causes defects in the printed image product, and adds cost to a need to change the blankets.
Conventional oil based inks are also well established market products in the lithographic printing market. However, conventional oil based inks have numerous deficiencies as well. For example, the time needed to develop the polymerization qualities of a coated conventional lithographic film is excessive.
One approach has attempted to combine the UV/EB lithographic inks with conventional oil based lithographic inks by applying UV/EB lithographic inks over conventional oil based inks. This approach has not been successful in that it typically results in a significant reduction in gloss and still requires a printed product be coated off-line after the conventional oil based ink is completely dry. Such an approach normally requires a delay of up to 72 hours for the conventional oil based inks to dry completely before the UV/EB coating can be applied off-line, a problem known as “dry back”. This is due to the fact that conventional oil based inks dry via a much slower oxidation mechanism requiring multi-step drying and curing. All of this results in added print production expense and production time constraints. Thus, there is a need for a UV/EB lithographic ink that maintains good physical and chemical resistance properties immediately upon and after curing.
Unfortunately, it has always been a challenge to formulate energy curable lithographic inks using acrylate monomers and oligomers as “building blocks”. A number of publications and patents have suggested the use of conventional resins such as rosin esters and alkyds in combination with acrylated monomers and oligomers to improve lithographic press performance. However, practical implementation of these ideas has not been widely demonstrated due to serious difficulties in producing stable mixtures of these materials so different in polarity and solubility. For example, UV lithographic inks have a limited water or fountain solution tolerance, which is also referred to as “water window.” Water window is defined as an ink's ability to sustain print density despite having an excessive amount of emulsified water. Inks with narrow water window tend to cause many lithographic press problems such as poor ink transfer which results in low print density, toning, piling, scumming, and excessive dot gain. The, overall behavior of lithographic ink regarding these factors defines its lithographic press performance.
Even where a lithographic ink as described above is successfully produced, it does not have a long shelf life and is prone to easy separation under severe printing press shear conditions, causing piling on the rollers and blankets. The prior art does not teach how to formulate a mixture of conventional and UV/EB curable lithographic inks having optimal lithographic press performance. It also does not teach how to effectively identify lithographic inks that offer optimal press performance from, for example, laboratory experiments.
Accordingly, there is a need to provide an energy curable lithographic ink composition having a long shelf life and optimal lithographic performance, including but not limited to a wider water window while still maintaining good lithographic performance on press, such as low dot gain, effective transfer of ink, effective ink trapping, good print contrast, good ink mileage and the absence of toning, scumming, picking, piling and dry back and other properties.